I've ordered a dozen frames from Zenni, and have had about a 50% success rate. My vision is pretty shit, so I get the maximum thinness to avoid as much weight on them as possible. As far as size and styling, I know around the mm length and width that fits my face, so every pair I've bought has "fit", it's just that certain styles don't work with my prescription.
Couple examples - these "sporty" sunglass style had a curved lens and with prescription made everything fucked up. I have no idea why they offered rx as an option with that style. Second example is the wire frames - they never bend back to have the lenses line up perfectly on my face, so it impedes my vision.
As far as build quality, if you get the $20-25 frames, especially plastic (which I tend to do because they're lighter), they're gonna be mostly cheap feeling. Every $50+ pair has been solid, even the plastic ones. Most of the pairs I've gotten have been between $100-125, the most thin lens, antiglare, oliophobic layer, which means the less themselves are $75-80.
As far as scratching, I've only ever done that to one pair, and it was because I'd set them on the side of a pool and stepped on them getting out when I was too drunk to realize what I was doing.
They do offer a safety glass option, which doesn't scratch nearly as easily, and when it does it isn't as blatantly obvious. I only know this because I know someone who had to use that option for their job.
If your glasses are falling off your face, it sounds like they're a) too heavy, b) too wide, c) screws are loose, d) frames are too big in general. My large lens frames give me the best vision, but fuck them things are always sliding down my face. Skinny frame with rubber earpieces also stick to your face better.
Last thing, I was able to use my HSA to buy mine, so I spent like $1200 to keep my HSA from capping out before the year ended. Never need to spend it on actually medical shit, so why not. I'm sure I could even get reimbursed $100 every two years by my VSP but that's not worth the effort. It's like pennies out of my check for the insurance and I just use it for the eye appointment to make sure I don't have eye cancer or whatever.
